The present invention relates to X-ray imaging equipment, and particularly to techniques for calibrating the dose rate of the X-rays emitted by this equipment.
Medical diagnostic X-ray imaging commonly involves fluoroscopic examination of a patient, such as during a catherization procedure. For this application the X-rays transmitted through the patient are converted to a visible light output image by an image intensifier tube. The output image is viewed by a video camera and displayed by a monitor enabling the medical personnel to observe the patient in real-time. The fluoroscopic image sequence can be simultaneously recorded on magnetic tape.
In the fluoroscopic mode, the X-ray exposure extends for a relatively long duration. As a consequence, the X-ray dose rate, and hence the X-ray tube current, must be maintained relatively low. U.S. government regulations require that during fluoroscopy, the dose rate shall not exceed ten Roentgens per minute (10 R/min.) at the plane where the X-ray beam enters the patient.
When fluoroscopy is not required the same X-ray equipment can be used to record images on a photographic medium. In this application the output of the image intensifier tube is recorded using a cine or photospot film camera. The X-ray emission in this mode of operation is pulsed to expose each frame of the film.
Regardless of the mode of operation, fluoroscopic or film, the equipment uses an automatic brightness control (ABC) system which regulates the X-ray emission to maintain the light output from the image intensifier at a substantially constant level. It should be noted that due to the different light sensitivities between the video and film cameras, the different modes of operation require different intensity levels of the output image. This control system insures readable images and are produced despite variation in the density of different portions of the patient's body which move into the exposure. U.S. Pat. No. 4,703,496 discloses one type of an automatic brightness control system.
In the fluoroscopy mode, the control system monitors the luminance of the video image as an indicator of the image intensifier's light output intensity (brightness). In the cine or photospot camera modes, a photodetector directly senses the light output from the image intensifier. The automatic brightness control system regulates the X-ray tube anode to cathode current, the anode to cathode voltage (KV) and the size of the camera apertures to maintain the level of light entering the respective camera substantially constant. However this regulation process is complicated by the interaction of these tube excitation parameters. The anode-to-cathode current is a function of primarily the cathode (or filament) temperature, while a constant voltage is applied across the anode and cathode. Although X-ray intensity and hence the image brightness, are directly proportional to the anode-to-cathode current, a non-linear relationship exists between the X-ray intensity and the anode-to-cathode voltage.
Previous brightness control systems, such as disclosed in the patent cited above, adjusted the excitation parameters on a priority basis. Initially the tube anode-to-cathode current is varied to keep the output light level constant. If that fails to maintain a constant image intensity or when further variation would exceed the maximum permitted dose rate, the voltage is adjusted. As a last resort the camera's iris aperture is opened to transmit more light to the camera. In addition in the fluoroscopic mode the video camera gain can be increased, but this has the negative side effect of increasing signal noise as well.
Before the X-ray system can be placed in operation, it must be calibrated for each selectable dose rate to account for variations in the characteristics of system components, such as the image intensifier, the video camera, and the X-ray tube. Previously, the calibration process involved placing a dosimeter in the plane at which the X-ray beam enters the patient. The X-ray equipment was then operated in the fluoroscopic mode at a selected dose rate. The camera's iris was manually adjusted until the control loop produced the selected dose rate as indicated by the dosimeter. This process was repeated for each selectable fluoroscopic dose rate. A similar manual calibration procedure was used to calibrate the brightness control loop for the cine or photospot camera operating modes.